This invention relates to a small antenna having a narrow HPBW (Half-Power Beam Width) in the horizontal plane that can be adapted, for example, to a third-generation (IMT-2000 system) six-sector wireless zone. This invention more particularly relates to an antenna that uses a plurality of non-powered metal conductors and has beam characteristics in the horizontal plane that are suitable for a six-sector wireless zone.
Repeated use of the same frequency in adjacent zones is a characteristic of a third-generation system, and the service area must be divided and the number of sectors increased in order to increase subscriber capacity. It is also known that narrowing the HPBW in the horizontal plane is more effective for increasing subscriber capacity than narrowing the angle of sector division (Reference: “Optimal Beamwidth of Base Station Antennas for W-CDMA” 1999 General Conference of The Institute of Electronics, Information, and Communication Engineers). In a six-sector wireless zone, since the division angle of one sector is 60°, an antenna having a HPBW in the horizontal plane that is narrower than 60° is needed in order to increase subscriber capacity.
Generally known methods for narrowing the HPBW in the horizontal plane involve enlarging the reflecting device. FIG. 11 shows an antenna in which the HPBW in the horizontal plane is set to 45° by a dipole antenna and a planar reflector. Dipole antennae 111 and 112 are arranged parallel to and in front of the planar reflector 110. The aperture width of the planar reflector 110 for making the HPBW 45° in the horizontal plane is 150 mm as found by a moment method when the central frequency used is 2 GHz, for example, and a length of one wavelength λ2G of 2 GHz is necessary.
By another well known method, the same effect as widening the antenna aperture width is obtained by placing a metal conductor near the antenna, and inducing an electric current in the metal conductor. FIG. 12 shows a 60° beam antenna in which metal conductors are placed on both sides of the antenna, and the HPBW in the horizontal plane is set to 45°. The dipole antennae 121 and 122 in front of the reflector 120 are arranged opposite each other and parallel to the planar reflector 120. Metal conductors 123 and 124 substantially equal in length to the reflector 120 in the longitudinal direction are arranged parallel to the dipole antennae 121 and 122 at a wider spacing than the spacing between the dipole antennae 121 and 122. These metal conductors 123 and 124 produce the same effect as widening the reflector 110 shown in FIG. 11, and the HPBW in the horizontal plane is set to 45°.
Another example described in Japanese Patent Application Laid Open No. 2004-15365 that uses a metal conductor is shown in FIG. 14. In the example shown in FIG. 14, a first metal wire 142 substantially equal in length to the radome of a multi-frequency common 120° beam antenna 140 is placed in a position at a distance S1 from the center of the beam antenna 140 in the direction ±90° with respect to the main radiation direction of the antenna 140, a second metal wire 143 shorter than the first metal wire 142 is placed in a position at a distance S2 nearer than distance S1 in the same direction, and the HPBW is narrowed to 90°.
The method for enlarging the reflecting device shown in FIG. 11 has drawbacks in that the already installed antenna is unusable. This, of course, necessitates replacing the antenna, which makes interruption of service unavoidable and places a burden on the user. When the reflecting device is enlarged, since the surface area blown by wind increases and the strength of the building material becomes an issue when the antenna is mounted on the rooftop of a building or the like, it may become impossible to install a desired antenna in some cases. Methods for enlarging the reflecting device therefore involve significant burdens both in service and economic aspects.
The method shown in FIG. 12 whereby the metal conductors 123, 124 are placed near the antenna has advantages in that the existing antenna can be used. However, the conventional method has drawbacks in that the back lobe level and side lobe levels increase when the HPBW is narrowed.
The solid line in FIG. 13 indicates the directional characteristics in the horizontal plane of the antenna shown in FIG. 12 in which the HPBW is narrowed using metal conductors. In FIG. 13, the angle of the main radiation direction of the antenna is set to 90°, and the axis scale is normalized so that the maximum value is 0 dB. The half bandwidth (−3 dB) for when the metal conductors 123, 124 of FIG. 12 are not present, indicated by the dashed line in FIG. 13, is 60°, but the half bandwidth is indeed 45°, as shown in FIG. 13, due to the effect of placing the metal conductors. However, the back lobe in the direction of 270° is increased by about 3 dB. The antenna gain in the 30° and 150° directions offset 60° from the main radiation direction is also at a level of about −13 dB, and lowering the gain of the back lobe and side lobes in order to decrease interference is desirable when the original purpose is considered, which is to increase the subscriber capacity by reducing interference to narrow the HPBW. It can hardly be said that adequate directional characteristics in the horizontal plane are obtained by conventional methods that use a metal conductor in this manner.